The present invention relates to a defect detection device used for an optical disc apparatus and the like for detecting a defect (point where normal write/read operation fails) on an optical disc.
In recent years, in computer systems in which the information amount to be handled has substantially increased, large-capacity, high-speed optical disc apparatuses permitting random access operation have come into widespread use as recording/playback apparatuses for information data. These apparatuses use optical discs such as CD-R (compact disc recordable), CD-RW (CD rewritable), DVD-R/RW (digital versatile disc recordable/rewritable) and DVD-RAM (DVD random access memory), for example, as recording media.
A defect detection device used for an optical disc apparatus as described above generally detects a defect on an optical disc by converging a light beam on the optical disc and detecting a change in the envelope of a signal corresponding to the intensity of light reflected from the optical disc, and outputs a defect detection signal indicating the presence/absence of a defect. The defect detection signal may be used as a signal for holding a preceding value in a servo circuit that controls tracking and focusing servo for the optical disc, or used to obtain an extraction signal for determining a recording-prohibited region of the optical disc using a CPU incorporated in the optical disc apparatus for various controls.
FIG. 9 is a block diagram showing a construction of a conventional defect detection device. FIG. 10 is a graph showing the waveforms of signals used in the defect detection device of FIG. 9. The operation of the defect detection device of FIG. 9 is as follows.
A light beam is converged on an optical disc, and a reflection signal AS corresponding to the intensity of light reflected from the optical disc is input into a variable gain amplifier 902. The variable gain amplifier 902 amplifies the reflection signal AS with a gain corresponding to a recording gate signal WTGT indicating that it is during recording of data into the disc or during playback of data from the disc, to obtain a predetermined amplitude, and outputs the resultant signal to a high-speed envelope detection circuit 940. By this use of the variable gain amplifier 902, it is possible to prevent the difference in the level of light reflected from the disc between during recording and during playback from being detected as a change in envelope.
The high-speed envelope detection circuit 940 detects the envelope of the input signal, and outputs the results to a differential circuit 906 and an integration circuit 960. The integration circuit 960 integrates the output of the high-speed envelope detection circuit 940, and outputs the results to the differential circuit 906. For example, assume that the envelope of the reflection signal AS abruptly changes due to a defect present on an optical disc. An envelope signal EM output from the envelope detection circuit 940, of which the time constant is small, has a waveform following the abrupt change of the envelope, as shown in FIG. 10. On the contrary, an output signal IS of the integration circuit 960 has a smoothly changing waveform, in spite of the abrupt change of the reflection signal AS, as shown in FIG. 10.
The differential circuit 906 outputs a differential signal DF corresponding to the difference between the envelope signal EM and the output signal IS of the integration circuit 960 to a comparator 908. The comparator 908 digitizes the differential signal DF using a signal SD output from a D/A converter 912 as a slice level, to generate and output a defect detection signal DD (see Japanese Laid-Open Patent Publication No. 2003-196853, for example).
However, the device of FIG. 9 fails to eliminate the level difference in reflected light between during recording and during playback due to a variation in the setting of the gain of the variable gain amplifier 902 and other reasons. Therefore, at a shift of the operation for an optical disc from recording to playback or from playback to recording, a level difference arises in the output AP of the variable gain amplifier 902 as shown in FIG. 10.
In particular, when the operation for an optical disc shifts from recording to playback, while the envelope signal EM output from the envelope detection circuit 940 follows the level difference of the reflection signal AS, it takes time for the signal IS, obtained by integrating the envelope signal EM, to follow the level difference. The differential signal DF is therefore higher than the signal SD, and thus a false defect detection signal DD is output over a considerable time period (pulse FS in FIG. 10). As a result, highly stable playback is not achieved. When the operation for an optical disc shifts from playback to recording, correct defect detection is unattainable over a considerable time period although a false defect detection signal DD will not be output.